The invention is specifically directed to a method and system for cleaning perforated, slotted and wire wrapped well liners which become plugged with foreign material by means of devices using high velocity liquid jets. Specifically, a method and system is employed with a tubing string that is non-rotating. It will be understood that in certain instances the inventive method and system can be applied to cleaning pipes in general and as used herein the term "pipe" shall include well liners.
In the well producing art, it is customary to complete wells, such as water, oil, gas, injection, geothermal, source, and the like, by inserting a metallic well liner adjacent a fluid-producing formation. Openings in the well liner provide passage-ways for flow of fluids, such as oil or water and other formation fluids and material from the formation into the well for removal to the surface. However, the openings, which, for example, may be slots preformed on the surface or perforations opened in the well, will often become plugged with foreign material, such as products of corrosion, sediment deposits and other inorganic or hydrocarbon complexes.
Since removal and replacement of the liner is costly, various methods have been developed to clean plugged openings including the use of jetted streams of liquid. The use of jets was first introduced in 1938 to directionally deliver acid to dissolve carbonate deposits. In about 1958 the development of tungsten carbide jets permitted including abrasive material in a liquid which improved the ability of a fluid jet to do useful work. However, the inclusion of abrasive material in a jet stream was found to be an ineffective perforation cleaning method in that it enlarged the perforation which destroyed the perforation sand screening capabilities.
More recently, Chevron Research Company, disclosed a method and apparatus for directionally applying high pressure jets of fluid to well liners in a number of U.S. patents. These patents are U.S. Pat. Nos. 3,720,264, 3,811,499, 3,829,134, 3,850,241, 4,088,191 which are herein incorporated by reference.
The assignee of the subject application developed a cleaning operation and device pursuant to the Chevron disclosures. The system employed a jet carrier of about six feet in length, having eight jet nozzles widely spaced along its length. The nozzles were threadably mounted on extensions which were in turn welded to the jet carrier. The jet carrier was attached to a tubing string that could be vertically reciprocated and horizontally rotated within the well bore. As the carrier was moved vertically and rotated adjacent the liner, the nozzles directed jet streams which contacted and cleaned the liner. This design developed a number of problems one of which was that there was no known relationship between the vertical and rotational speed which would assure efficient and complete liner coverage by the fluid streams.
In an attempt to solve these problems, Applicant developed its own jet carrier assembly fully described in co-pending application, Ser. No. 195,303, filed Oct. 7, 1980, now U.S. Pat. No. 4,349,073, which is herein incorporated by reference. This assembly has between about 8 and 16 nozzles spaced along its length. An equation is used to determine the jet stream track pattern against the liner for a jet tool having a given nozzle number and spacing and which is rotated and moved vertically at selected speeds. The spacing between the tracks is then calculated from this track pattern. Comparing the spacing with the known width of a jet stream determines the amount of coverage the streams provide on the liner. Using this equation, a set of rotational and vertical speed of a constant ratio were determined which would provide jet streams having theoretical double coverage over all points on the liner when using 16 nozzles.
Although the design was a major advance in the art, it did not attempt to relate the rotational and vertical speeds to the diameter of the liner. To solve this problem, Applicant developed a system in which the energy needed to clean the liner is determined and related to the factors which the operator can control in the field. After determining the energy needed to clean the liner, the power drop between the nozzle and the liner is calculated as a dependency of the stand-off distance, ie. the distance of the jet from the liner. Knowing the power drop, one can determine the total energy of the streams at the nozzles needed to produce the required cleaning energy at the liner. The rotational speed and maximum vertical speed are then calculated which will produce this total energy for a given liner size and given plugging condition. This system is fully disclosed in co-pending application Ser. No. 308,582 filed Oct. 5, 1981 which is herein incorporated by reference.
Although Applicant's systems described above are quantum advances in the art of well cleaning, they employ a high pressure rotating swivel, which is, in turn, rotatably connected to a tubing string. The fact that the tubing string is freely rotatable permits rotation of the carrier at speeds which ensure complete liner coverage by the jet streams as the carrier is moved vertically. In short, these carriers are not applicable to non-rotating tubing strings.
A safe and economically efficient alternative to jointed tubing or conventional rig is the coiled tubing rig. In general, coil tubing is a continuous string of small diameter tubing that can be run into the well from a large reel without the necessity of making joint connections. This operation, therefore, saves rig time and is usually more economical to employ. Many workover operations can be completed quickly and efficiently by using coiled tubing instead of the convention rigs. Theoretical burst pressures of typical coiled tubing are on the order of between 11,400 psi and 14,500 psi. This is well below the operating pressures for hydraulic jet cleaning.
The problem with employing coiled tubing rigs with hydraulic jet well cleaning is that because the coiled tubing is wound on a reel, the tubing string is not rotatable in the conventional manner such as by rotating swivel. Applicant is not aware of any hydraulic jet well cleaning operations employing non-rotating tubing strings such as formed of coiled tubing. A "non-rotating" tubing string as used herein shall mean a string which is not conveniently rotatable.
As a result, a strong need exists for a method and system for cleaning well liners which can be employed with non-rotating tubing strings and which will clean the particular foreign material present in a controllable, economical field operation.